3,143 research outputs found
A Pre-Protostellar Core in L1551
Large field surveys of NH3, C2S, 13CO and C18O in the L1551 dark cloud have
revealed a prolate, pre-protostellar molecular core (L1551-MC) in a relatively
quiescent region to the northwest of the well-known IRS 5 source. The kinetic
temperature is measured to be 9K, the total mass is ~2Msun, and the average
particle density is 10^4-10^5 cm^(-3). L1551-MC is 2.25' x 1.11' in projection
oriented at a position angle of 133deg. The turbulent motions are on the order
of the sound speed in the medium and contain 4% of the gravitational energy,
E_{grav}, of the core. The angular momentum vector is projected along the major
axis of L1551-MC corresponding to a rotational energy of 2.5E-3(sin
i)^(-2)|E_{grav}|. The thermal energy constitutes about a third of |E_{grav}|
and the virial mass is approximately equal to the total mass. L1551-MC is
gravitationally bound and in the absence of strong, ~160 microgauss, magnetic
fields will likely contract on a ~0.3 Myr time scale. The line profiles of many
molecular species suggest that the cold quiescent interior is surrounded by a
dynamic, perhaps infalling envelope which is embedded within the ambient
molecular gas of L1551.Comment: 27 pages, 7 figures, ApJ accepte
O stars and Wolf-Rayet stars
Basic information is given about O and Wolf-Rayet stars indicating how these stars are defined and what their chief observable properties are. Part 2 of the volume discussed four related themes pertaining to the hottest and most luminous stars. Presented are: an observational overview of the spectroscopic classification and extrinsic properties of O and Wolf-Rayet stars; the intrinsic parameters of luminosity, effective temperature, mass, and composition of the stars, and a discussion of their viability; stellar wind properties; and the related issues concerning the efforts of stellar radiation and wind on the immediate interstellar environment are presented
Molecular Emission Line Formation in Prestellar Cores
We investigate general aspects of molecular line formation under conditions
which are typical of prestellar cores. Focusing on simple linear molecules, we
study formation of their rotational lines by radiative transfer simulations. We
present a thermalization diagram to show the effects of collisions and
radiation on the level excitation. We construct a detailed scheme (contribution
chart) to illustrate the formation of emission line profiles. This chart can be
used as an efficient tool to identify which parts of the cloud contribute to a
specific line profile. We show how molecular line characteristics for uniform
model clouds depend on hydrogen density, molecular column density, and kinetic
temperature. The results are presented in a 2D plane to illustrate cooperative
effects of the physical factors. We also use a core model with a non-uniform
density distribution and chemical stratification to study the effects of cloud
contraction and rotation on spectral line maps. We discuss the main issues that
should be taken into account when dealing with interpretation and simulation of
observed molecular lines.Comment: Accepted for publication in Ap
Solar science with the Atacama Large Millimeter/submillimeter Array - A new view of our Sun
The Atacama Large Millimeter/submillimeter Array (ALMA) is a new powerful
tool for observing the Sun at high spatial, temporal, and spectral resolution.
These capabilities can address a broad range of fundamental scientific
questions in solar physics. The radiation observed by ALMA originates mostly
from the chromosphere - a complex and dynamic region between the photosphere
and corona, which plays a crucial role in the transport of energy and matter
and, ultimately, the heating of the outer layers of the solar atmosphere. Based
on first solar test observations, strategies for regular solar campaigns are
currently being developed. State-of-the-art numerical simulations of the solar
atmosphere and modeling of instrumental effects can help constrain and optimize
future observing modes for ALMA. Here we present a short technical description
of ALMA and an overview of past efforts and future possibilities for solar
observations at submillimeter and millimeter wavelengths. In addition, selected
numerical simulations and observations at other wavelengths demonstrate ALMA's
scientific potential for studying the Sun for a large range of science cases.Comment: 73 pages, 21 figures ; Space Science Reviews (accepted December 10th,
2015); accepted versio
Re-examining Larson's Scaling Relationships in Galactic Molecular Clouds
The properties of Galactic molecular clouds tabulated by Solomon etal (1987)
(SRBY) are re-examined using the Boston University-FCRAO Galactic Ring Survey
of 13CO J=1-0 emission. These new data provide a lower opacity tracer of
molecular clouds and improved angular and spectral resolution than previous
surveys of molecular line emission along the Galactic Plane. We calculate GMC
masses within the SRBY cloud boundaries assuming LTE conditions throughout the
cloud and a constant H2 to 13CO abundance, while accounting for the variation
of the 12C/13C with Galacto-centric radius. The LTE derived masses are
typically five times smaller than the SRBY virial masses. The corresponding
median mass surface density of molecular hydrogen for this sample is 42
Msun/pc^2, which is significantly lower than the value derived by SRBY (median
206 Msun/pc^2) that has been widely adopted by most models of cloud evolution
and star formation. This discrepancy arises from both the extrapolation by SRBY
of velocity dispersion, size, and CO luminosity to the 1K antenna temperature
isophote that likely overestimates the GMC masses and our assumption of
constant 13CO abundance over the projected area of each cloud. Owing to the
uncertainty of molecular abundances in the envelopes of clouds, the mass
surface density of giant molecular clouds could be larger than the values
derived from our 13CO measurements. From velocity dispersions derived from the
13CO data, we find that the coefficient of the cloud structure functions,
vo=sigma_v/R^{1/2}, is not constant, as required to satisfy Larson's scaling
relationships, but rather systematically varies with the surface density of the
cloud as Sigma^{0.5} as expected for clouds in self-gravitational equlibrium.Comment: Accepted by ApJ. Newest version includes modifications from the
refere
Spectrum of Optically Thin Advection Dominated Accretion Flow around a Black Hole: Application to Sgr A*
The global structure of optically thin advection dominated accretion flows
which are composed of two-temperature plasma around black holes is calculated.
We adopt the full set of basic equations including the advective energy
transport in the energy equation for the electrons. The spectra emitted by the
optically thin accretion flows are also investigated. The radiation mechanisms
which are taken into accout are bremsstrahlung, synchrotron emission, and
Comptonization. The calculation of the spectra and that of the structure of the
accretion flows are made to be completely consistent by calculating the
radiative cooling rate at each radius. As a result of the advection domination
for the ions, the heat transport from the ions to the electrons becomes
practically zero and the radiative cooling balances with the advective heating
in the energy equation of the electrons. Following up on the successful work of
Narayan et al. (1995), we applied our model to the spectrum of Sgr A*. We find
that the spectrum of Sgr A* is explained by the optically thin advection
dominated accretion flow around a black hole of the mass M_bh=10^6 M_sun. The
parameter dependence of the spectrum and the structure of the accretion flows
is also discussed.Comment: AAS LaTeX file; 26 pages; 12 ps figures; to be published in ApJ. PDF
files are obtainable via following anonymous ftp.
ftp://ftp.kusastro.kyoto-u.ac.jp/pub/manmoto/preprint/spec_sgrA.tar.g
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